The long-term goal of this project is to explain the ionic permeability and active transport mechanisms which underlie retinal function. In this period, we propose to examine the membrane distribution of Na-K pumps in photoreceptors and glial fibers from several species (frogs, turtles, and rabbits). Our primary experimental approach here will be quantitative autoradiography of 3H-ouabain, a specific marker of Na-K pumps. In addition, we propose to examine the hypotheses that acetylcholine is the transmitter released by photoreceptors (turtles) and the GABA is the transmitter mediating horizontal cell feedback onto photoreceptors (frog and turtle). In particular, we will attempt to learn if synaptic pedicels of photoreceptors accumulate acetylcholine which can be released upon depolarization and if photoreceptors possess binding sites for high affinity GABA agonist. Our primary experimental approach will be quantitative autoradiography of isolated photoreceptors. The answer to these questions will provide a better understanding of the basic physiology of retinal function and thereby provide a rational framework for understanding its diseases.